The present invention relates to detecting corrosion in metal junctions, and, in particular, relates to detecting corrosion in metal junctions where corrosion affects current flow.
Corrosion is a significant problem for both military and commercial systems. Corrosion can cause structural failure of the corroded member. In electronic equipment, corrosion can distort or disrupt the electrical signal that propagates through a metallic connector, that is, a conductor susceptible to corrosion. Corrosion is also a significant source of Electromagnetic Interference ("EMI") that can disrupt nearby systems. Damaging levels of corrosion can be hard to detect because the corrosion may be inaccessible or imperceptible on visual inspection. The effect of corrosion on electrical transmission is generally not linear.
The failures caused by corrosion in the path of an electrical signal are often intermittent. Factors such as temperature, humidity, amount and type of use, mechanical stresses on the point of corrosion--all influence both the degree of corrosion and its effect on current flow. The rate of growth of corrosion also depends on what is corrosive in the surrounding medium. Thus, even if one knows that corrosion is present, neither the rate of growth of corrosion nor the time a system will start to fail can be predicted.
Current techniques that address corrosion fall into two categories. The first includes techniques that detect corrosion directly. The chief technique is visual inspection. X-rays can also detect the presence of foreign material, sometimes including corrosion, in metal junctions.
Visual inspection may be inadequate to identify adverse degrees of corrosion on metallic structures. The corrosion can be inaccessible, or it can be invisible to the naked eye. In electromagnetic systems even slight surface corrosion at metallic junctions can impede current flow. When one disconnects at these junctions to inspect them, friction of the corroded spots against each other, or against an uncorroded piece, can temporarily remove the corrosion. This process explains why turning a battery in its slot can make the battery work better temporarily. The corrosion on the battery (or on the connections to it) was invisible, but the corrosion limited current flow until the surfaces rubbed together. Similarly, in other electromagnetic systems, the corrosion could easily be erased temporarily when connectors are unmated prior to inspecting them. For a technique to be effective at identifying corrosion that inhibits current flow, it must detect corrosion prior to unmating the connectors.
X-ray methods are expensive, and the foreign materials they detect in metal junctions may not be corrosion. Further, x-ray methods are not practical for routine testing of field equipment.
The second category includes techniques that attempt to detect the adverse effects caused by corrosion. Two techniques predominate. A Loop Resistance Tester ("LRT") can detect very small resistance in electrical cables and connectors. A Time-Domain Reflectometer ("TDR") can detect discontinuities in electrical conductors, some of which could be caused by severe corrosion.
A LRT cannot identify corrosion as the cause of increased resistance in the current path. Since corrosion problems are often intermittent, resistance measurements generally fail to identify a corrosion problem as it develops.
Thus there exists a need for apparatus and methods for detecting non-intrusively the presence of corrosion in metal to metal junctions.